Date
/Volcanic activity (and human response)
March 20, 1980
/At approximately 3:45 PM a magnitude 4.2 earthquake occurred just northwest of the summit of Mount St. Helens at shallow depth.Seismologists were uncertain as to whether or not these first earthquakes were related to volcanic activity. They decided to deploy additional seismometers in order to better monitor future activity.
March 21, 1980
/Earthquake activity increased. Over the next week, continued concern over earthquake-triggered avalanches would lead the USFS to close the mountain above the tree line.
March 22, 1980
/A second earthquake larger than magnitude 4.0 occurred beneath Mount St. Helens.Seismologists determined that the current earthquake sequence could be precursory to volcanic activity.
March 23, 1980
/The Seismologists determined that the current earthquakes could be a precursor to volcanic activity.
March 24, 1980
/Earthquake activity continued to increase, with 10 earthquakes larger than magnitude 3.0 and 4 larger than magnitude 4.0 (including one of magnitude 4.7).
March 25, 1980
/Increasing number of earthquakes occurred throughout the day.
March 26, 1980
/The University of Washington recorded the 100th earthquake larger than magnitude 3.5, as well as 7 larger than magnitude 4.0. Depths for most quakes were less than 3 miles.
March 27 1980
/At approximately 12:30 people near the volcano reported hearing a loud "boom," which probably marked the first sighted explosion. Following the explosion a crater about 200 to 250 feet wide was reported near the summit. The evacuation within a 15 mile radius from the volcano was ordered.
March 28, 1980
/Phreatic explosions began at 3AM and lasted nearly 2 hours. A cloud of ash and steam rose more than a mile above the volcano. Scientists flew over the volcano to collect samples of escaping gases. They found that sulfur dioxide was the main species of sulfur present. This was the first indication that a high-temperature magmatic source was releasing gas during explosions from somewhere inside the volcano.
March 29, 1980
/A second crater formed to the west of the first and was visible on the morning of the 29th. Steam and ash was vented from this new crater. More earthquakes were recorded – one was 4.4.
March 30, 1980
/Explosions, ash falls and earthquakes continued. A carnival-like atmosphere prevailed around Mount St. Helens – many people wanted to see the volcano.
March 31, 1980
/Both craters enlarged as explosions continued. The frequency of earthquakes decreased while the number of larger earthquakes increased. A state of emergency was declared. Harry Truman refused to leave his home on the south shore of Spirit Lake.
April 1, 1980
/Several plumes from explosions reached altitudes of 20,000 feet. Ongoing explosive activity caused the two craters to merge into one single crater over 600 feet across. Two earthquakes of magnitude 4.7 occurred, as well as the first weak harmonic tremor. This led scientists to suggest that there is an increased possibility of an eruption involving magma.
April 2, 1980
/More plumes of steam and ash were seen – one reached an altitude of 20,000 feet. The 2nd of April turned out to have more earthquakes than any other in April.
April 3, 1980
/Explosions sent plumes upwards to altitudes of 17,000 feet. The crater is now approximately 1500 feet wide and 300 feet deep. Tourists continue to come see the volcano putting themselves and others at risk.
The Eruption of Mount St. Helens, Summary of the Events March 20 – May 17, 1980
April 4, 1980
/More explosions, episodes of harmonic tremors and earthquakes were recorded.
April 5, 1980
/At least 3 explosions sent ash to about 15,000 feet.
April 6, 1980
/It was another day – much the same as others – though there no harmonic tremors recorded.
April 7, 1980
/The crater's dimensions now measured approximately 1700 feet long by 1200 feet wide and 500 feet deep. The overall seismicity remained the same – though harmonic tremors were recorded.
April 8, 1980
/Morning eruptions lasted over 4 hours. It was the longest uninterrupted sequence of explosions to date. Scientists were not sure what would happen next.
April 9, 1980
/There were more explosions and release of sulfuric gas during the day. Tourist traffic would be a problem if an evacuation was needed.
April 10, 1980
/More earthquakes, light ash falls continued through the day. The cost of monitoring and protecting the mountain was increasing.
April 11, 1980
/A magnitude 4.9 earthquake was recorded in the afternoon, and along with the one on March 28 is the strongest to date. The tiltmeter on the north side of the mountain showed some evidence of swelling.
April 12, 1980
/Instruments detected ground swelling near Goat Rocks. A 27-minute episode of harmonic tremor recorded in the afternoon ended with a magnitude 4.5 earthquake. This would be the last episode of harmonic tremor recorded for nearly one month.
April 13, 1980
/More explosions of ash and steam and earthquakes continued.
April 14, 1980
/More earthquakes but not much happening.
April 15, 1980
/Only one explosion and no changes in the ground tilt were recorded.
April 16, 1980
/Limited activity – seismic activity reached the lowest point for April.
April 17, 1980
/Two earthquakes of 4.6 recorded. Scientists became concerned about the possibility of an avalanche on the north flank.
April 18, 1980
/Steam plumes rose to about 500 feet above the crater and seismicity remained moderate. The volcano settled into a "holding pattern" over the next several days.
April 19, 1980
/Local scientists familiar with the volcano recognized that the upper north flank of the volcano was moving outward, but disagreed on why. Some thought the summit graben subsidence was to blame, others magmatic inflation pushing from within the volcano. Low levels of earthquakes were recorded.
April 20, 1980
/No explosions occurred but seismicity reached the highest point in two weeks.
April 21, 1980
/Seismicity returned to low levels.
April 22, 1980
/Small steam bursts were noted and some changes were recorded by the tiltmeters.
April 23, 1980
/Limited activity was observed. There was some evidence that the volcano might be inflating due to increased magma pressure from below. Some parts of the volcano are 250 feet higher than in 1979.
April 24, 1980
/Fumaroles continued to vent steam – low earthquake levels were noted.
April 25, 1980
/Continuing steam venting and earthquake activity were recorded. The bulge was growing and it was becoming a concern in the case of an avalanche.
April 26, 1980
/Limited activity on the mountain.
April 27, 1980
/Fumaroles continued to vent steam. The bulge was growing – 15 – 20 feet in the last four days.
April 28, 1980
/Small puffs of steam were visible and there ws continued deformation of the north flank of the mountain.
April 29, 1980
/Aerial photographs revealed that the bulge was about one mile long and one half mile wide. Scientists believed the bulge represented the most serious potential hazard. The most serious concerns were about an avalanche of snow, ice, and rock coming down on Spirit Lake at more than 100 miles per hour.
April 30, 1980
/The volcano remained quiet, but the bulge continued to expand northward.A Red Zone was established whose boundary ranged from 3 to 7 miles out from the peak. Access in this zone was restricted to scientists, law enforcement, and other officials. A Blue Zone was established. Access into this area was restricted during daylight hours to loggers and property owners with special permits.
May 1, 1980
/Steam venting continued.
May 2, 19980
/Earthquakes continued. Warm rock areas were detected in the middle of the bulge.
May 3, 1980
/The mountain continued to steam without explosions and the bulge continued to grow.
May 4, 1980
/The volcano remained quiet. The bulge continued to move outwards by as much as 5 feet per day.
May 5, 1980
/Strong emissions of ash have been uncommon for the past three weeks. It was speculated that the instability of the bulge might result in a massive landslide.
May 6, 1980
/Earthquake activity continued at about the same level as during previous weeks. The bulge continued to push outwards at 4 to 5 feet per day.
May 7, 1980
/Explosions resumed after a two week break. Steam and ash plumes reached 13,000 feet. The bulge continued to push outwards.
May 8, 1980
/Steam was observed in the crater. Harmonic tremor was also recorded, the first since April 12.
May 9, 1980
/Explosions continued. Continuous steaming in the crater was observed.
May 10, 1980
/Steam and ash emissions were visible once the weather cleared. Surveys of the bulge showed that the outward motion had slowed to less than 3 feet per day.
May 11, 1980
/Small bursts of steam and ash were emitted.
May 12, 1980
/Small explosions continued. Observers noted a cluster of fumaroles on the western rim of the crater. A magnitude 5.0 earthquake triggered an 800 foot wide avalanche down the north flank.
May 13, 1980
/Steam and ash explosions decreased in frequency.
May 14, 1980
/The north flank continued to bulge outwards at about 5 feet per day.
May 15, 1980
/The bulge continued to push outward at about 5 feet per day. Seismic activity included an earthquake of magnitude 4.8.
May 16, 1980
/Steam and ash explosions decreased in frequency. The bulge continued to push outward at about 5 feet per day. There have been no records of harmonic tremor since May 8. Steam continued to emanate from the summit area. The shape of the volcano has changed considerably during the past 10 days. The bulge continued to appear highly broken and distorted.
May 17, 1980
/The mountain remained quiet. Seismic activity reached the lowest level for May.
May 18, 1980
/At 0832 on May 18, a complex earthquake (M=5.1) shook the volcano, probably causing (but possibly caused by) a huge, 2.7-cubic-kilometer-landslide that in three different blocks successively removed the bulge and upper 400 meters of the volcano leaving a 600-meter-deep crater 2 kilometers wide rim-to-rim.